Paul Nicholls has revealed both Big Buck's and Zarkandar suffered midweek scares.
The pair are set to be odds-on favourites on the opening day of the John Smith's Grand National meeting at Aintree but the former, winner of the Ladbrokes World Hurdle for the third successive season, needed a blood test to confirm his participation.
Writing in his exclusive column at www.betfair.com/paulnicholls the champion trainer said: "Monday and Tuesday he went off his grub, so on Tuesday we gave him a blood test.
"Thankfully, it came back fine and he ate up this morning, so we are back on track."
Brilliant JCB Triumph Hurdle winner Zarkandar has been battling a mouth abscess to add to Nicholls' problems.
"He has had a bad abscess in his mouth and we have been constantly bathing it. He hasn't missed any work but of course it has been very sore, though he is a tough buggar," he continued on Betfair.
"But thank god it burst yesterday (Tuesday) morning, so touch wood it will have done his chances no harm at all.
"To put it into context though, I must stress that both horses have continued to work well throughout the minor scares, and are fine."
Nicholls also provides a horse-by-horse guide to the rest of his Thursday team in the column.